The present invention relates to a printing quality inspection apparatus which inspects the printing quality of an image printed on opaque paper such as white paper available as a general printing sheet, vapor deposition paper having undergone a vapor deposition process on its surface, or a transparent film.
In general, opaque paper such as white paper has a diffusely reflecting surface, whereas vapor deposition paper has a specularly reflecting surface. A conventional printing quality inspection apparatus which inspects whether an image printed on such opaque paper or vapor deposition paper is printed in a normal state includes two types of illumination light sources: a dark-field illumination light source and a bright-field illumination light source, as disclosed in Japanese Patent Laid-Open No. 11-108637 (literature 1). When an image of a printing product printed on an opaque printing paper sheet is to be detected by the conventional printing quality inspection apparatus, the dark-field illumination light source is turned on to use dark-field illumination light. On the other hand, when an image printed on vapor deposition paper is to be detected by this printing quality inspection apparatus, both the dark-field illumination light source and the bright-field illumination light source are turned on to use both dark-field illumination light and bright-field illumination light, respectively.
Japanese Patent Laid-Open No. 11-108635 (reference 2) proposes another printing quality inspection apparatus including a light diffusing member which is formed from a translucent material and is provided between an object to be inspected and an illumination light source. In this printing quality inspection apparatus, illumination light emitted by the illumination light source scatters in various directions upon being transmitted through the light diffusing member. The target surface of the object to be inspected is illuminated with the scattered illumination light at a nearly uniform illuminance, and captured by an image capture camera.
In recent years, a transparent film is often used as a printing sheet to print an image on the transparent film. To inspect whether an image is printed on the transparent film in a normal state, the operator extracts printed samples for every predetermined number of sheets, and visually inspects them for a printing failure. Therefore, a heavy burden is inflicted on the operator. Also, when a printing failure occurs in a printing product which is not extracted as a sample, a defective printing product is shipped without being detected. Moreover, even if the operator becomes aware of a printing failure, defective printing products are produced in large quantities, thus wasting printing materials.
In the conventional printing quality inspection apparatus disclosed in literature 1, two types of light sources: dark- and bright-field light sources are necessary, thus leading to an increase in cost. Also, it is necessary to operate a switch for switching the process involved, in accordance with whether a printing product printed on opaque paper or that printed on vapor deposition paper is to be inspected, thus inflicting a heavy burden on the operator. Furthermore, if the operator is unaware of erroneous detection because he or she has forgotten to switch the process involved, large amounts of printing materials are wasted.
On the other hand, in the printing quality inspection apparatus disclosed in literature 2, the light diffusing member is inserted between the object to be inspected and the illumination light source. Therefore, due to a shortage of illumination light which impinges on the target surface, an image printed on transparent paper cannot reliably be detected.